In recent years, it has become apparent that conventional methods of generating electricity will soon be insufficient to meet the world's ever-growing need for electric power. Several factors, including the pollution which results from the combustion of fossil fuels, the dangers associated with the operation of nuclear reactors, and the limitations inherent in the traditional hydroelectric as well as in the more modern solar energy approaches to the generation of electricity, have encouraged the development of alternative sources of electric power, such as the wind turbine generator.
Wind turbines convert wind energy to electrical energy in a manner analogous to the way in which the windmills of Western Europe converted wind energy to mechanical energy for pumping water or operating grinding mills. A wind turbine generally includes a rotor which is mounted for rotation near the apex of a tower approximately 18 to 50 meters in height. The rotor acts as the prime mover for an electrical generator which provides power through transformers and substation controlled connections, to the local utility power grid.
Generally, wind energy projects include the installation of large numbers of wind turbine generating systems at locations having favorable wind conditions. Several of these so-called "wind farms" are located in the state of California.
In late 1986 it became apparent that the various wind energy projects using wind turbines to generate electrical energy suffered from a major problem in that the turbines were not receiving the amount of wind energy that was projected based on the initial wind studies that were conducted. There have been many reasons advanced for this shortfall of wind energy. Little can be done about the wind itself, with the exception of understanding the available resource better (by using direct measurement and analysis). However, new rotor blades, designed to take better advantage of the available wind resource, provide an opportunity for energy increase.
Another major problem which has been associated with wind energy projects is mechanical failure in existing wind turbines. It has been found that the direction of the wind is not always along the rotational axis of the rotor. Off-axis wind components cause mechanical loads on the blades that were not adequately considered when the original blades were designed. Particularly, when the wind rises along a slope to a wind turbine placed at the top of the slope, it creates an additional "yawing" (side to side) load. This is sometimes called "vertical flow." When the wind comes in from either side ("yawed flow") it creates an additional "pitching" load (bottom to top or top to bottom, depending on the yawed flow direction). Although the wind turbines have "active yaw systems" which are designed to rotate in response to changes in wind direction so that the rotor always faces the direction from which the wind is blowing, it has been found in practice that the rate of yaw rotation is slow compared to the rapid and variable changes in wind direction which are common in nature. These additional loads are causing major damage in turbine systems in areas such as California.
A factor contributing to mechanical failure of existing wind turbine blades is that the aerodynamic loads, which begin at the tip, are integrated along the length of the blade. Therefore, longer blades (especially those producing more energy due to increased airfoil efficiency) will have higher loads at the base or root, thus making the design of the structure more critical.
Another major problem associated with existing wind turbine blades is leading and trailing edge cracking. In prior designs the top half of the blade is formed in one mold, while the bottom half is formed in another mold. Then, both halfs are sealed to a spar. Alternatively, the skin is "hinged" either fore or aft. Defects in the leading edge can ruin airfoil efficiency. Such defects are more likely to occur along a joint between separately molded parts, and may occur either during manufacture or during continued use. It is often necessary to rework or make "repairs" to newly manufactured parts, or to replace blades in the field due to premature failures.
One prior approach to increasing energy output is simply to increase the swept area through the use of devices called "hub extenders." These devices fit between the base of the existing blades and the hub, thus increasing the length of the blades and therefore the swept area. However, these devices increase the total weight of the rotor system, increase aerodynamic and gravity loads on the mechanical components of the wind turbine and also require a double set of attachment bolts, thus introducing another point of potential component failure.
The Vestas V-15 and Vestas V-17 wind turbine generators are found at various wind farm sites in California. These machines are typical of the Danish turbines used in many wind energy projects Both of these machines are three blade, upwind, active yaw (turning into the wind) machines running nearly synchronously at approximately 51 rpm. The V-15 has a 75kw electric induction generator (nameplate 65kw) and the V-17 has a 110kw generator (nameplate 90kw). The V15 uses 7.5M blades of a basic NACA 44xx airfoil series and the V-17 uses 8.5M blades of the same series. The average thickness-to-chord ratio for the series used is approximately 0.18 and thus the typical airfoil cross section is an NACA4418. The existing blades are highly twisted, with the twist changing by about 18.degree. from root to tip. In use, these NACA 4418 blades are also subject to considerable fouling by dirt and insect debris which reduce operating efficiency and make frequent washing a necessity. The blades also weigh on the order of 1,000-1,200 pounds each, which is considered excessive for their function by modern technology standards. These blades, or very similar designs, are used in thousands of turbines installed in California.